Anytime your browser attempts to download a .php file instead of processing it, it usually means that the server was unable to process the script before a timeout happened, and it attempts to send you the results of what it was able to do, but as a download instead of as proper browser output.
Often it's a timeout problem that causes that.
Timeouts can happen from a number of causes:
- processing the page logic is taking too long
- a resource required in order to process the page logic is being silently denied
- the server is overly busy and cannot process the script normally
Sometimes such a problem is caused by corrupt or incomplete or missing files caused by a bad FTP upload.
The best place to start when having installer problems is to ensure that the /includes and /zc_install folder are all complete: all files present, none are 0-bytes, and no failures during upload.
Use a true dedicated FTP program such as FileZilla to do mass uploads.
Do not use design/development suites like dreamweaver or any MS product to do mass uploads.